There are several known mixing and kneading methods and devices commonly used for the manufacture of carbon paste.
One known device, historically the oldest but still in use today, consists primarily of a closed and substantially horizontal vat, with two rotating arms of a particular shape referred to as “Z arms” turning inside it in opposite directions, each respectively rotating around one of two axes that are substantially parallel and substantially in the same horizontal plane. This type of machine processes and prepares the paste in batches: each machine delivers a fixed quantity of prepared paste at fixed intervals, as opposed to a continuous processing and preparation method in which the paste is prepared at a constant rate.
FIGS. 1a and 1b respectively show a plan view with the vat open at the top, and an elevated side view with a vertical partial cross-sectional view, of an embodiment of such a machine. The vat (1) receives the paste components, pitch as binder, and the mixture of coke and recycled carbon-containing waste (also called “dry materials” to distinguish it from the pitch which is initially liquid or becomes so when hot). The vat (1) has a double wall with a heat transfer fluid circulating inside, at a temperature that is typically between 200 and 300° C. The coke and the recycled carbonaceous waste thus may, in a first mixing step of the method, be heated or maintained at their temperature if they are introduced hot into the vat (1). The pitch can be introduced in liquid form or in solid form. Two “Z arms” (2) rotate in opposite directions and often at different speeds, mixing and kneading or crushing the pitch and the dry materials. Each of the arms is driven by one of two respective gear motors (4) to rotate around its respective longitudinal axis XX or X′X′. When the mixing and kneading method is complete, a door (3) in the base of the vat (1) is opened to unload the prepared paste. The vat (1) and the two gear motors (4) are installed on a common frame (5). After the vat (1) is emptied, a new cycle can begin.
For example, in a typical machine of this first type, the vat (1) has a capacity of 5500 kg of paste, the mixing and kneading cycle lasts about an hour, and the nominal motor power is 190 kW. Such a machine weighs about 22000 kg and occupies a volume of 40 m3. These characteristics show that this type of machine does not provide good performance: the output relative to the mass and the bulk of this machine is low, while the power involved is very high for the production results. In fact, mixing and kneading with “Z arms” (2) is inefficient. This low power is compensated for by longer mixing, at the expense of the output rate. In addition, the gear motors (4) have to have a high reserve capacity in order to handle all the various situations, because the machine does not have any means of regulating the cycle: the vat (1) is filled in a fixed manner and the paste temperature can only be modified very slowly. The actual average consumption is much lower than the nominal capabilities of the motors of the gear motors (4) used.
A machine of this type and the paste manufacturing method used by this machine, in which the paste components are mixed and kneaded under mechanical pressure (compressing the paste components in the vat), are described in patent document FR 2 022 697, which can be referred to for more information.
Today, the most commonly used device for manufacturing carbon paste by mixing and kneading its components, binder, and fillers as indicated above, is a horizontal worm mixer-kneader. An alternating axial displacement is superimposed on the rotational movement of the worm. This back and forth movement improves the mixing and kneading of the paste components. A known machine implementing this mixing and kneading method is represented in a schematic axial cross-sectional view in FIG. 2a, and in a larger-scale transverse cross-sectional view in FIG. 2b. 
A worm (6) resting on two supports (7) and primarily consisting of a shaft with a helicoidal thread protruding outwards from the shaft is rotated by a control unit (8). The worm (6) is housed inside a cylindrical casing (9) equipped with an inlet (10) and an outlet (11). The raw materials, heated beforehand to the mixing temperature, typically 180° C., are introduced into the kneader-mixer at the inlet (10). The thread of the worm (6) is not continuous; it is interrupted for a portion of the circumference. FIG. 2b shows a typical arrangement where the thread consists of three identical sections (12), spaced at regular intervals 120° apart from each other per turn. Fixed teeth (13) are installed on the cylindrical casing (9) facing these defined open areas, projecting inwards from said casing towards the axis of rotation of the worm (6). FIGS. 2c and 2d are perspective drawings of the thread sections (12), spaced apart from each other around the rotational axis of the screw (6) by 120°, and protruding from the shaft (14) of the worm (6), FIG. 2c showing only an axial section of said shaft; the fixed tooth (13) with its foot for anchoring it in the cylindrical casing (9) providing a better view of the arrangement.
The continuous rotation of the worm (6) imparts a horizontal translational movement to the component materials of the paste, which draws them towards the outlet (11). Simultaneously, the worm (6) has a back-and-forth movement within the casing (9). This back-and-forth movement is synchronized with the continuous rotation of the worm (6) around its axis, so that the areas or sections (12) of the thread of the worm (6) can pass between the fixed teeth (13) of the casing (9) without interfering with these teeth. The continuous rotation, as well as the advancing component of the back-and-forth motion, therefore ensure that the materials advance from the inlet (10) to the outlet (11). This advance is hindered by the obstacles posed by the fixed teeth (13) of the casing (9), and by the backward movement of the worm (6). These combined axial and radial displacements efficiently crush the component materials of the paste while applying shear to the mixed materials, between the fixed teeth (13) and the threaded areas or sections (12) of the worm (6).
A machine of this type operates continuously. The component materials of the paste are introduced at a constant and controlled rate and exit at the same rate. At the outlet (11), two remote-controlled doors control the aperture size of the outlet from the mixer-kneader. It is thus possible to vary the degree to which the machine is filled, in order to maintain a constant level of power consumption. A commonly used machine of this type is capable of a paste output of 35 tons/hour, with a nominal power of 350 kW. By regulating the power, the yields are much better than those of the “Z arm” mixer-kneaders described above with reference to FIGS. 1a and 1b. 
By their very design, the type of machine in FIGS. 2a and 2b contains little paste at any given moment, typically about a ton which is the average weight of the anodes produced. For this reason, these machines are often criticized for not homogenizing the paste sufficiently to eliminate variations in the quality of the raw materials used to form the paste.
A carbon paste manufacturing machine of this second type is described in patent document U.S. Pat. No. 4,652,226.
A third type of machine used in carbon paste manufacturing has recently been introduced for kneading the paste: a mixer-kneader with a tilted rotating drum, in which a mixing and kneading tool rotates similarly to a mill. The different components of such a machine are diagrammed in the attached FIG. 3.
A cylindrical drum (15), its axis tilted from the vertical, is rotated by a gear motor (16). A rotary tool (17) fitted with several paddles (18) is rotated around the tilted axis of the drum (15) by another gear motor (19). As it rotates, the drum (15) brings the materials (20) to the rotary tool (17) where the component materials of the paste are subjected to shear and dispersion to encourage mixing. A fixed scraper (21) dislodges the paste from the tilted side walls and from the bottom of the drum (15) so the paste falls vertically back into itself. There is a door (22) of variable size placed at the center of the drum (15), for discharging the prepared paste.
The machine provides a continuous flow during operation. The quality and quantity of the supply of materials to be mixed and kneaded is controlled. The amount of materials kept inside the drum (15) can be controlled by the door (22) opening. Of course, the power consumption varies with the level to which the drum is filled (15). It is therefore possible to regulate the power consumption in order to optimize the quality of the mixture and the yield from the machine. Similar results are also obtained by controlling the rotational speed of the rotary tool (17).
A typical machine, illustrated in FIG. 3, capable of a continuous paste output of 35 tons per hour holds 2500 kg of paste and has a nominal power of 200 kW. On average, the capacity of the machine corresponds to the production of two and a half anodes, which means a good level of homogenization.
A machine such as the one illustrated in FIG. 3 is commonly used to supplement other types of mixer-kneaders, for additional blending of the paste and in particular to cool it. It is best to do the kneading at a high temperature, typically between 180 and 200° C., where the pitch viscosity is lower in order to optimize pitch penetration into the coke particles and recycled carbonaceous waste. However, as the electrode production method applied downstream from the mixer-kneader requires paste at a lower temperature, a paste cooling step is required. Water is therefore introduced into the drum (15) at a controlled rate during the mixing process, to bring the paste to the desired temperature, typically from 140 to 160° C.
This machine, illustrated in FIG. 3 and commonly used as a cooler, only recently began to be installed as a primary mixing and kneading machine for the preparation of carbon paste, particularly anode paste.
From this brief presentation of the prior art, it can be seen that the preparation of carbon paste for aluminum production electrodes uses machines of various technologies, with their advantages and disadvantages. These widely differing methods used by the different machines provide similar end results, a lack of effectiveness of the mixing and kneading actions being offset by the processing time, which leads one to conclude that the methods for mixing and kneading carbon paste for electrodes are not yet optimized. Mixing at high temperatures is a given, however, if a good result is to be achieved quickly. But, due to electrode production requirements downstream from the mixers/kneaders, it is necessary to cool the paste at the end of the kneading. For this reason, modern facilities for manufacturing carbon paste for electrodes are always equipped with two devices or machines installed sequentially: a mixer-kneader which is one of the three above types and a cooler, usually a rotating tilted drum as illustrated in FIG. 3 and described above.